This invention relates to an exhaust gas turbine supercharger for an internal combustion engine and, more particularly, to a variable-capacity turbine supercharger provided with a capacity control mechanism for controlling supercharging capacity in accordance with the operational condition of the engine.
In for example, U.S. Pat. No. 3,270,495, a conventional variable-capacity exhaust gas turbine supercharger of the aforementioned type has a turbine housing divided into, first and second scrolls by an inner wall thereof, and the nozzle portion of each scroll is opened in opposition to the circumferential surface of a turbine blade. The first scroll is joined directly to an exhaust manifold, and the second scroll is joined to the exhaust manifold via a control valve. The control valve is adapted to control the capacity by closing the inlet of the second scroll when the flow rate of exhaust gas is low, and by opening the inlet of the second scroll when the flow rate of the exhaust gas is high.
The aforementioned supercharger is designed so that the pressure of exhaust gas flowing at a high flow rate with the engine rotated at a high speed can be reduced, by reducing the cross-sectional area of a passage in a so-called open scroll connected directly to an exhaust manifold, securing a supercharging pressure when the exhaust gas flows at a low flow rate with the engine rotated at a low speed, and increasing to as great an extent as possible the cross-sectional area of a passage in a so-called closed scroll. However, no consideration is given to the areas of the openings of the nozzle portions of the scrolls. If the cross-sectional area of the passage in the closed scroll is decreased, a choking phenomenon occurs in the nozzle portion thereof to cause the turbine efficiency to decrease, and the exhaust gas pressure to increase.